1. Field of the Invention
The invention is in the field of well logging, in which measurements taken in a borehole are used in searching for and exploiting valuable underground resources such as oil and gas. It is particularly directed to a method and a system for natural gamma radiation logging, in which a log is derived of the radiation detected in selected energy windows and is converted into a log of selected subsurface materials, such as thorium, uranium and potassium. Yet more specifically, the invention is directed to deriving a log of the selected subsurface materials which is substantially corrected for errors due to factors such as radiation emitting materials (e.g. potassium, in the form of potassium chloride) in the borehole fluid, and radiation absorbing materials, such as barite and/or hematite, in the mud filtrate.
2. The Prior Art
In prior art natural gamma radiation logging, a tool capable of detecting gamma radiation in each respective one of several energy windows is passed through a selected borehole interval, and a record is made of the gamma rays detected within the respective windows. The gamma rays are emitted in the decay of subsurface materials such as thorium, uranium and potassium (Th, U, K), each of which emits a characteristic spectrum resulting from the emission of one or more gamma ray at various energies. The tool output is converted to a log of the concentrations of Th, U, K at the respective borehole depth levels.
The Th, U, K log is important in searching for and exploiting underground resources because it is believed that these materials appear in nature with a discernible relationship to geology and rock morphology. The log is particularly useful in the exploration for and exploitation of oil and gas resources because it is believed that the concentrations of Th, U, K taken individually or in combination are a good indication of previously unavailable information as to the presence, type and volume of shale or clay in the formations surrounding the borehole.
In practice, the nature of the logging process makes the detected spectra continuous, with poor energy resolution and poor counting statistics. Nevertheless, there are known techniques for usefully estimating and logging the Th, U, K concentrations.
The difficult measurement conditions in Th, U, K logging have been made yet more difficult by the common use of borehole fluids (drilling mud) containing potassium chloride (KCl) and weighting materials which are strong absorbers of gamma rays, such as barite and/or hematite (hereafter B). These borehole fluids stabilize the borehole by reducing clay and shale hydration and provide various other benefits. However, the KCl in the borehole fluid emits its own gamma radiation whose contribution is merged with that of potassium in the undisturbed formations surrounding the borehole, while the strong absorber B in the same borehole fluid can significantly reduce the gamma radiation flux from the surrounding formations. Typically, the drilling and logging environment makes it impossible or impractical to measure the concentration of KCl and B in the mud at the time the borehole logging commences.
Also, it is known that barite in the mud has a significant effect in nuclear (scattered gamma radiation) logging. See Seeman U.S. Pat. No. 3,900,733 and references cited therein, for a discussion of techniques attempting to correct for the barite effect. It is also known that KCl in the mud filtrate has a significant effect in natural gamma radiation logging. See Cox, J. W. et al, "The Effect Of Potassium-Salt Muds On Gamma-Ray, And Spontaneous Potential Measurements," SPWLA 1976, and references cited therein. Additional uncertainties are introduced by the fact that relatively few gamma rays can be detected in the respective energy windows at a given borehole depth because the tool must move through the borehole at a sufficiently high speed to allow drilling or production activities to resume as soon as possible, and by the fact that the tool response changes as a function of borehole size.
Some aspects of known gamma radiation well logging are discussed in Marrett, G. et al, "Shaly Sand Evaluation Using Gamma Ray Spectrometry, Applied to the North Sea Jurassic," Proc. SPWLA 17th Annual Logging Symposium, Jun. 9-12, 1976, and Serra, O. et al, "Theory, Interpretation and Practical Applications of Natural Gamma Ray Spectroscopy," Proc. SPWLA 21st Annual Logging Symposium, Jul. 8-11, 1980, and additional information can be found in Chevalier et al., U.S. Pat. No. 3,976,878 and Moran et al U.S. Pat. No. 3,521,064. As discussed in the above cited documents, all of which are incorporated by reference herein, it is possible to convert the output of a natural gamma radiation logging tool having several (e.g. five) energy windows into a log of thorium, uranium and potassium concentrations (Th, U, K), in essence by subjecting the tool output to a filter characterized by a 3.times.3 or 3.times.5 matrix which can be empirically derived-such as by passing the tool through a test borehole containing known concentrations of Th, U, K arranged to approximate the effect of homogeneous beds of infinite depth and radial extent and recording the windows responses. If W designates the radiation detected in five energy windows at a given borehole depth level i.e., U=[U.sub.1, U.sub.2, U.sub.3, U.sub.4, U.sub.5 ], and X designates the thorium, uranium and potassium concentrations at the same depth level, i.e., Y=[Th, U, K], then the relationship between the windows measurements U and the concentrations Y (when no environmental effects are present) can be described by: EQU U=SY+E (1)
where S is defined by a 5.times.3 tool sensitivity matrix which is unique to a given tool and can be empirically derived by passing the tool through a borehole containing known concentrations of Th, U, K in idealized beds, and E=[e.sub.1, . . . , e.sub.5 ] denotes the statistical errors which are due to the Poisson nature of gamma ray detection. What is of interest normally is the concentrations of Th, U, K as a function of the radiation detected in the windows, and therefore what is of interest is the relationship: EQU Y=M U (2)
where M is defined as a 3.times.5 matrix relating the concentrations of the three materials to the radiation detected in the five energy windows at a given depth level in the borehole. M can be found through a least squares technique relating known concentrations Th, U, K to measured radiation in the five energy windows for given test conditions. The matrix M need be found only once for a given logging tool.
In the known technique, a log of the Th, U, K concentrations is derived by evaluating the relationship (2) at each depth level in the borehole. Additional aspects of borehole effects correction are discussed in Ellis, D., "Correction of NGT Logs for the Presence of KCl and barite Muds", Proc. SPWLA 23rd Annual Conference, Jul. 6-8, 1982 (NGT is a trademark of Schlumberger). As discussed in that document, which is incorporated by reference herein, the gamma radiation emitting subsurface materials logs may be corrected for at least one of: (i) the gamma ray emitter potassium chloride (KCl) in the borehole fluid, and (ii) a gamma ray attenuator (absorber) in the borehole fluid, e.g. barite and/or hematite.
It has also been proposed in U.S. Pat. No. 4,568,829 a method for improving the log of Th, U, K concentrations based on the recognition that the concentrations log can be filtered not in a fixed manner but adaptively, in accordance with changes with borehole depth in the detected radiation and an understanding of the nature of the logging process. In particular, the Th, U, K concentrations estimate for a previous depth level by an amount determined through applying a filter (constructed for the given depth level) to a combination of: (i) the radiation detected in the five energy windows for the given depth level and (ii) an estimate for the radiation in the five energy windows derived by applying the tool sensitivity matrix to the concentrations estimate for the previous depth level.
U.S. Pat. No. 4,542,292 describes a method wherein is derived a log of gamma radiation detected in selected energy windows, e.g. five, for a selected borehole interval, and converting it into a log of the selected materials, e.g. Th, U, K, which is substantially corrected for at least one of: (i) a gamma ray emitter in the borehole fluid, e.g. potassium salts and (ii) a gamma ray attenuator in the borehole fluid, e.g. a strong attenuator such as barite and/or hematite. In a particular embodiment, the concentrations of the three materials (Th, U, K) are related through an empirically derived logging tool sensitivity matrix to five corrected window measurements. The known method in the '292 patent is based on assumptions which alter its reliability. First, the KCl concentrations and B corrections found as described above for the individual depth levels in the borehole typically differ significantly from one depth level to another. The only way to overcome this difficulty is to assume that the KCl is typically well mixed in the borehole fluid, and that its concentration should be reasonably constant throughout the well. In the case of the strong absorber, its effective absorption is mainly related to the diameter of the borehole and could be significantly perturbed by variable mudcake build-up.
The above depicted prior art show detrimental drawbacks. One correction factor has to be applied to each of the respective energy windows (or channels). Furthermore, the correction factor differs from one radioactive element to the other. Accordingly, the calculation and computation steps are time consuming and therefore limit the ability of the system to compute and display the data in real time. The real time ability could be improved by decreasing the logging speed, but at the cost of increasing the duration of the logging run, which is not desirable businesswise. Another constraint complicates further the situation, that is a phenomenon called "statistics" which alters the detection; the higher the logging speed, the bigger the statistics effects.
Consequently, there is a need for an improved method of correction for borehole effects on natural gamma ray logs.